Flush doors are simple door designs that have plain facings on both sides. They can be interior (more commonly) but also exterior. Solid flush doors are, by definition, heavier than the alternatives, but are usually a more suitable option when their function is to form a passage between two rooms. They are often used where space is at a premium, and would probably be placed towards the “functional” end of the door design spectrum rather than the “style” end but as with anything a little imagination goes a long way! They may be kept deliberately simple in appearance to highlight other features nearby. Flush doors may have a solid, hollow or stave core.
If the core is solid – low density particle board or foam often being used to fill the space within the door completely –
it may be supplemented with a type of laminate or plywood on each side. Good for exterior doors as they provide more insulation and strength.
If the core is hollow (i.e. where perhaps two sections of plywood have been attached to each side of a frame) some support may be built into the frame (such as a lattice or honeycomb support system made of corrugated cardboard).